U.S. patent number 4,467,187 [Application Number 06/292,413] was granted by the patent office on 1984-08-21 for automatic focusing device which inhibits focusing when a low contrast scene is sensed.
This patent grant is currently assigned to Canon Kabushiki Kaisha. Invention is credited to Takashi Amikura, Tokuichi Tsunekawa.
United States Patent |
4,467,187 |
Tsunekawa , et al. |
August 21, 1984 |
Automatic focusing device which inhibits focusing when a low
contrast scene is sensed
Abstract
In an automatic continuous focusing device, two detected images
of an object formed by a distance detection optical system with a
relative distance therebetween associated with the object distance
are scanned, the obtained scanning signals are quantized to
generate quantized picture element data, the relative distance
between the two detected images is detected on the basis of the
quantized picture element data, and an imaging optical system to be
focused on the object is controlled according to the detection
result. The device also provides for invalidating the control of
the imaging optical system when the results of detection of the
quantized condition of the scanning signals indicate that the
quantized condition of the scanning signals for at least one of the
two detected images is not sound or reasonable.
Inventors: |
Tsunekawa; Tokuichi (Yokohama,
JP), Amikura; Takashi (Tokyo, JP) |
Assignee: |
Canon Kabushiki Kaisha (Tokyo,
JP)
|
Family
ID: |
14661921 |
Appl.
No.: |
06/292,413 |
Filed: |
August 13, 1981 |
Foreign Application Priority Data
|
|
|
|
|
Aug 22, 1980 [JP] |
|
|
55-115412 |
|
Current U.S.
Class: |
250/204;
250/201.6; 396/116; 396/125 |
Current CPC
Class: |
G02B
7/30 (20130101) |
Current International
Class: |
G02B
7/30 (20060101); G03B 003/10 () |
Field of
Search: |
;250/201,204,209,578
;354/25R,25A,31F,402,404,406-409 ;356/1,4 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Nelms; David C.
Assistant Examiner: Westin; Edward P.
Attorney, Agent or Firm: Fitzpatrick, Cella, Harper &
Scinto
Claims
What we claim is:
1. An automatic focusing device comprising:
sensing means for sensing an image of an object and for generating
a plurality of signals therefrom;
control means for generating a control signal;
focusing means for focusing an optical system on the basis of the
plurality of signals from said sensing means, said focusing means
focusing the optical system only when said focusing means
continuously detects the control signal for a predetermined time
period; and
inhibiting means having a detection period shorter than the
predetermined time period, said inhibiting means detecting each of
the plurality of signals from said sensing means in each detection
period and inhibiting the focusing means when the plurality of
signals from said sensing means are at predetermined
conditions.
2. A device according to claim 1, wherein said inhibiting means
inhibits the operation of said focusing means until at least the
termination of the next detection period.
3. A device according to claim 2, wherein said control means
produces the control signal on the basis of the plurality of
signals from said sensing means.
4. A device according to claim 3 further comprising:
quantizing means for quantizing the plurality of signals from said
sensing means and for inputting the plurality of quantized signals
into said control means and said inhibiting means.
5. A device according to claim 4, wherein said quantizing means
compares the plurality of signals from said sensing means with a
plurality of reference signals and generates a first quantized
signal and a second quantized signal.
6. A device according to claim 5, wherein said inhibiting means
inhibits the operation of said focusing means when only one of the
first and second quantized signals is detected.
7. A device according to claim 6, wherein said inhibiting means
includes first latch means for latching the first quantized signal
and second latch means for latching the second quantized
signal.
8. A device according to claim 7, wherein said first and second
latch means are reset simultaneously before the start of each
detection period of said inhibiting means.
9. A device according to either claim 3 or 8, wherein said focusing
means includes drive means for driving the optical system, adding
means for accumulating the control signal from said control means,
and switching means for starting the operation of said drive
means.
10. A device according to claim 9, wherein said adding means
includes an electric condenser for integrating the control
signal.
11. A device according to claim 10, wherein said drive means
includes a motor of which both ends are grounded when a signal for
inhibiting the operation of said focusing means is provided from
said inhibiting means.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an automatic focusing device and,
more particularly, to an automatic continuous focusing device
wherein two detected images of an object formed by a distance
detecting optical system with a relative distance therebetween
associated with the object distance are scanned by a scanning
means, the scanning signals thus obtained are quantized by a
quantizing means to generate quantized picture element data, the
relative distance between the two detected images is detected by
operation means on the basis of the quantized picture element data,
and an imaging optical system to be focused on the object is
controlled by a control means for focusing it according to the
detection result.
2. Description of the Prior Art
Some automatic focusing devices of the type described above have
been proposed. For example, in the specifications of U.S. Pat. Nos.
4,004,852 and 4,078,171 (corresponding Laid-open Japanese patent
application No. 51-153433), examples of an automatic focusing
device are disclosed according to which two detected images formed
by a distance detection optical system of stationary base line
distance measuring unit type having no moving parts with a relative
distance therebetween corresponding to the object distance are
scanned by a solid image pickup element such as a line type image
sensor, the scanning picture element signals thus obtained are
converted into binary signals (quantized) by a binary encoder
(quantizing means) to generate binary picture element data
(quantized picture element data) consisting of signals of logic
values "0" and "1", the relative distance between the two detected
images is detected by a digital arithmetic means according to the
binary picture element data to obtain an object distance, the data
on the object distance is compared with data on the position of an
imaging optical system to be focused on the object on the optical
axis to generate control signals, and the imaging optical system is
controlled by a control means including drive means such as a motor
for focusing it on the object in response to the control signals.
In particular, in the device disclosed in the U.S. Pat. No.
4,004,852, a method is adopted according to which data on an image
of a first field of view as a reference with respect to an object
for detection of an object distance and data on an image of a
second field of view wider than the first field of view (number of
data handled for the first and second fields of view are different
and the number of data for the second field of view is greater than
the other) are obtained, and a position of an image part in the
second field of view corresponding to the image in the first field
of view or most resembling it is detected to thereby obtain
distance data with respect to the infinite object distance as a
reference.
Unlike the device disclosed in U.S. Pat. No. 4,004,852 described
above which adopts the method of comparing data on the position of
the imaging optical system with the obtained absolute distance
data, U.S. patent application Ser. No. 944,974 filed on Sept. 22,
1978, now U.S. Pat. No. 4,305,657 (corresponding to British Patent
Application Publication No. 2009553) of the present assignee
discloses an example of an automatic focusing device according to
which part of the system associated with the second field of view
is movable and is finely moved on a solid pickup element in
association with the control of the imaging optical system so that
the image part of the second field of view corresponding to the
image of the first field of view or resembling it may be situated
at a predetermined position, whereby focusing of the imaging
optical system may be accomplished.
In a device of this type, particularly since the scanning signals
of the detected images of an object are quantized to generate the
quantized picture element data and the distance detection or the
focus detection is performed according to this quantized picture
element data, correct detection may not be achieved when the
contrast of the object is low, resulting in very unstable control
of the imaging optical system and occasionally in very erratic
operation. When the contrast of an object is low, the possibility
increases that the binary data which is obtained by slicing at a
predetermined threshold level may all be converted to the same
data, that is, the possibility of conversion to signals of a single
logic value "1" or "0" increases. When such a phenomenon occurs,
correct detection becomes impossible. Especially when a solid
pickup element is used as has been described hereinabove,
non-uniformity of the sensitivity of the photosensor elements also
becomes a big factor, contributing to the phenomenon described
above.
Especially with a camera for continuous photography such as an 8 mm
movie camera or a video camera, the subject changes continuously.
Therefore, when an automatic focusing device as described above is
mounted to a camera of this type, and when the subject changes from
an object of high contrast to an object of low contrast or of solid
color, correct detection may be performed with the object of high
contrast but correct detection may or may not be performed with the
object of low contrast or of solid color, resulting in very
unstable control operation of the photographic lens and
occasionally very erratic operation.
SUMMARY OF THE INVENTION
The present invention has been made in consideration of this and
has for its primary object to solve these problems associated with
the conventional automatic focusing device of the type wherein the
images are scanned and the scanning signals are quantized.
It is another object of the present invention to provide an
automatic focusing device which prevents unstable or erratic
operation of the focusing control which may otherwise be caused due
to unreasonable quantization into the quantized picture element
data which is attributable to the low contrast of an object or the
like.
It is still another object of the present invention to provide an
advantageous improvement which solves the problems as described
above in an automatic continuous focusing device of the type
wherein two detected images of an object formed by a distance
detection optical system with a relative distance associated with
the object distance are scanned by a scanning means, the scanning
signals thus obtained are quantized by a quantizing means to
generate quantized picture element data, the relative distance
between the two detected images is detected by operation means
according to the quantized picture element data, and the imaging
optical system to be focused on the object is controlled by a
control means for focusing it according to the detection
result.
In order to achieve the above and other objects, the present
invention provides an automatic focusing device of the type which
performs image scanning and quantization of the scanning signals
characterized in that a means is further incorporated for
invalidating control of the imaging optical system by the control
means for automatic focusing when the quantization of the scanning
signals for the detected images is not sound or reasonable, after
detecting the quantized condition of the scanning signals obtained
with a scanning signal quantizing means.
In association with the characteristic construction of the present
invention, as preferred embodiments of the present invention, the
description of which is to follow, are proposed examples of an
automatic focusing device of the construction according to which
the quantizing means performs quantization of the signals according
to different first and second quantized references, computation for
automatic focusing is performed according to quantized picture
element data generated based on the first quantized reference,
detection of the quantized condition of the scanning signals
obtained with the quantizing means is performed by an erratic
operation preventive means according to quantized picture element
data generated based on the second quantized reference, and the
second quantized reference is lower than the first quantized
reference. These improved constructions are further effective for
preventing unstable automatic focusing.
These and other objects and features of the present invention will
become apparent from the following description when taken in
conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
The embodiments of the present invention will be described with
reference to the accompanying drawings in which:
FIG. 1 is a schematic block diagram showing one embodiment of the
present invention;
FIG. 2 is a model view showing the case wherein the automatic
continuous focusing device shown in FIG. 1 is mounted in a camera
for continuous photography such as an 8 mm movie camera or a video
camera;
FIG. 3 is a circuit diagram showing the construction of the main
part of the signal processing and control circuit shown in FIGS. 1
and 2 which is particularly related to the present invention, the
construction of the motor drive circuit, and the construction of
the erratic operation preventive means according to the improvement
provided by the present invention; and
FIG. 4 is a timing chart for explaining the operation of the
circuit of FIG. 3.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 is a schematic block diagram of one embodiment of the
present invention wherein symbol OPS denotes a distance detection
optical block (optics) of stationary base line distance measuring
unit type having mirrors M1 and M2, lenses Ll and L2, and a prism
PR with reflecting surfaces M3 and M4; and SP denotes a
one-dimensional solid pickup element. An image of an object OB1 or
OB2 is focused through the optical block OPS on the solid pickup
element (pickup) SP. An image IM1 formed via the mirror M1, the
lens L1, and the reflecting surface M3 is a reference image, and an
image IB1 or IB2 obtained via the mirror M2, the lens L2, and the
reflecting surface M4 is an actual image. The object distance to
the object OB1 or OB2 is measured by detecting the relative
distance between the reference image IM1 and the actual image IB1
or IB2 by a signal processing and control circuit (processor) PCC.
Focus detection data is obtained by comparing this distance data
with data on the position of a photographic lens (lens) LS on its
optical axis (output data from a lens position data output means
LPD). The focus detection data is supplied to a motor drive circuit
MDC through output terminals MU1, MU2, ML1 and ML2. The
photographic lens LS is placed at the focal point with respect to
the object OB1 or OB2 by a lens drive motor (motor) MT through the
motor drive circuit (driver) MDC. Symbol SLF denotes a control
terminal of an erratic operation preventive means incorporated in
the signal processing and control circuit according to the present
invention and is connected to a control part within the motor drive
circuit MDC.
FIG. 2 shows an embodiment wherein the automatic continuous
focusing device as described above is mounted in a camera for
continuous photography such as an 8 mm movie camera or a video
camera. The same reference characters are used to refer to like
elements in FIG. 1. Referring to FIG. 2, symbol G1 denotes a pinion
gear mounted on a rotational shaft of the motor MT, G2 is a focus
ring drive gear mounted on a focus ring of the photographic lens
LS, and LPD is the lens position data output means such as a gray
code plate, a digital encoder or the like for supplying the control
data for the photographic lens LS to the signal processing and
control circuit PCC. The signal processing and control circuit PCC
compares the data on the object distance obtained by processing the
output of the solid pickup element SP with the data on the position
of the photographic lens LS on its optical axis supplied from the
lens position data output means LPD. As a result of this
comparison, the signal processing and control circuit PCC outputs
focus detection data consisting of signals of high and low levels,
that is, focus control signals, through its output terminals MU1,
MU2, ML1 and ML2. In response to these focus control signals, the
motor drive circuit MDC drives the motor MT for achieving focusing
by controlling the position of the photographic lens LS with
respect to the object.
Describing the function of the object distance detection in the
signal processing and control circuit PCC, in the optical block
OPS, the system consisting of the mirror M1, the lens L1, and the
reflecting surface M3 is a reference system for the object. An
image of the object formed via this reference system, that is, the
reference image, is formed at substantially the same position on
the solid pickup element SP irrespective of the object distance. On
the other hand, the system consisting of the mirror M2, the lens
L2, and the reflecting surface M4 is an actual system for referring
to the object at a position spaced apart from the reference system
by a predetermined base line distance in the lateral direction. The
image of the object formed by the actual system, that is, the
actual image is formed at a position corresponding to the object
distance on the solid pickup element SP; the actual image is formed
at a position shifted to the left in FIG. 1 when the object is far,
and it is formed at a position shifted to the right when the object
is near. Therefore, in the linear arrangement of the sensor
elements in the solid pickup element SP, a predetermined number of
sensor elements sufficient for detection of the reference image
(the range of the depth of field determined in this manner will be
referred to as the reference field of view) is available for the
reference system, and a predetermined number of sensor elements
sufficient to cover the range of movement of the actual image,
which is thus greater than the former predetermined number for the
reference field of view, (the range determined in this manner will
be referred to as an actual field of view) is available for the
actual system. The positions of the image in the actual field of
view and the image in the reference field of view are detected by,
for example, utilizing the focal point corresponding to an infinite
object distance as a reference, whereby detection of a distance
using this reference point may be accomplished. For this purpose,
the digital correlation method is adopted herein. The scanning
picture element signals output from the solid pickup element SP are
first converted into (quantized) binary signals of logic values "1"
or "0" based on a threshold value determined by the peak value of
the scanning picture element signals for the reference field of
view. Among the binary picture element signals, those binary
picture element signals corresponding to the image of the reference
field of view and those of the actual field of view are stored in a
memory circuit comprising shift registers or the like. Thereafter,
the binary picture element signals for the image of the reference
field of view and the binary picture element signals for the image
of the actual field of view are compared with each other bit by bit
from the locations corresponding to the infinite distance to form
sets of continuous picture elements which are equal in number but
are sequentially shifted from each other by one picture element. At
each comparison operation, the number of coincidences is counted
until the maximum number of coincidences, that is, until maximum
correlation is obtained. Then, the number of comparisons made until
this maximum number of coincidences is obtained, that is, the data
on the position of the image part which resulted in maximum
correlation in the actual field of view, is obtained. Thus, this
image part which resulted in maximum correlation is considered as
the image part corresponding to the reference image, so that data
on the position of this image part in the actual field of view may
be obtained as distance data utilizing the focal point
corresponding to an infinite object distance as a reference.
The signal processing and control circuit PCC functions to detect
the object distance in the manner as has been described above. The
distance data described above is obtained as digital data of a
predetermined bit number. On the other hand, the lens position data
output means LPD outputs lens position data as digital data of the
same number of bits, taking the infinite distance focal point as a
reference. Then, the signal processing and control circuit PCC
compares both data and outputs logic signals indicative of
out-of-focus (the focal point is deviated to the front or to the
back of the depth of field) or correct focusing from its output
terminals MU1, MU2, ML1 and ML2.
Referring to FIG. 3, description will now be made on the
construction of the main part of the signal processing and control
circuit PCC particularly related to the present invention, the
construction of the motor drive circuit MDC, and the construction
of the erratic operation preventive means incorporated in these
circuits according to the present invention. The same reference
characters are used to refer to like elements in FIGS. 1 and 2.
In the solid pickup element SP shown in FIG. 3, SE denotes a
photosensitive picture element part and DE denotes a
non-photosensitive dummy picture element part. When reading the
output of the solid pickup element SP, the signal processing and
control circuit PCC first detects and holds, at a dark current
signal detecting and holding circuit DTC, dark current signal data
obtained at the dummy picture element part DE in response to a
pulse .phi.D from an operation and control circuit (controller)
DCC. The signal processing and control circuit PCC subsequently
subtracts, at a differential amplifying circuit DA, the dark
current signal components from picture image data (the scanning
picture element signals) obtained at the photosensitive picture
element part SE to thereby obtain picture image data (the scanning
picture element signals) VF which is compensated for the dark
current. Among the picture image data VF, a peak value Vp of the
scanning signals from the picture element range corresponding to
the reference field of view is determined in response to a pulse
.phi.p from the operation and control circuit DCC, is detected by a
peak detecting circuit PDC, and is then held by a peak value
holding circuit PHC according to a pulse .phi.H from the operation
and control circuit DCC. The peak detecting circuit PDC is reset in
response to a pulse .phi.D from the operation and control circuit
DCC. The peak value Vp is divided by resistors R14 and R15 and the
picture image data VF is quantized by a quantizing circuit QC
taking the value ##EQU1## as the threshold level. Although the
quantizing circuit QC may comprise an A/D converter, a comparator
or the like, a comparator for binary data having the value obtained
from the peak value Vp as a threshold level is used in this
embodiment. RDC denotes a judging circuit for judging whether or
not the peak value Vp is within a suitable range. When the peak
value Vp is outside the suitable range, the charge storing time of
the solid pickup element SP is controlled by a group of pulses
.phi.SP in response to the output IT from the judging circuit RDC
so that the peak value Vp may fall within the suitable range.
The picture image data VF is converted into signal levels of logic
values "1" or "0" according to its level at the quantizing circuit
QC relative to the threshold level based on the peak value Vp of
the scanning signals of the reference field of view (e.g.,
0.8.times.Vp). The quantized picture image data QD thus obtained
are input to the operation and control circuit DCC which governs
the functions of object distance detection by the digital
correlation method described above and the focusing of the
photographic lens LS. The quantized picture image data QD are also
input to a quantized condition judging circuit (judger) as a
detecting part of the erratic operation preventive means according
to the improvement provided by the present invention, the quantized
condition judging circuit QDC comprising inverters IN1 and IN2, AND
gates AN1 to AN3, RS flip-flops FP1 and FP2, and a D flip-flop FP3.
The quantized condition judging circuit QDC judges whether or not
the quantized condition is reasonable or not, more specifically,
whether or not all of the binary picture element signals for the
reference field of view and the control field of view are "1" or
"0".
In the motor drive circuit MDC, symbols R1 to R10 denote resistors;
Tr1 and Tr2, pnp transistors; and Tr3 and Tr4, npn transistors, the
transistors Tr1 to Tr4 constituting a bridge circuit to control
forward and reverse rotation of the drive motor MT for driving the
photographic lens. Symbols D1 and D2 denote diodes for facilitating
braking for stopping the motor MT, D3 and D4 denote reverse
rotation preventive diodes, and Tr5 and Tr6 denote control
transistors for the transistors Tr1 and Tr2. Delay capacitors C1
and C2 are disposed at bases of the transistors Tr5 and Tr6 to
delay the turning on and off of the transistors Tr5 and Tr6 by a
certain length of time. The turning on and off of the transistors
Tr5 and Tr6 may otherwise be shortened by discharging the delay
capacitors C1 and C2 through transistors Tr7 and Tr8, thus driving
the photographic lens LS in a stable manner.
In the motor drive circuit MDC, symbol IEC denotes a lens control
invalidating circuit as a control part of the erratic operation
preventive means according to the improvement provided by the
present invention. In this lens control invalidating circuit
(invalidater) IEC, symbols R11 to R13 denote resistors; D5 to D7,
diodes; Tr9, a pnp transistor; and Tr10 and Tr11, npn transistors.
These parts constitute a circuit for instantaneously stopping the
drive motor MT for driving the photographic lens in response to an
output SLF of high level from the quantized condition judging
circuit QDC.
E denotes a power source (battery) and SW denotes a power source
switch.
The operation and control circuit DCC has functions of controlling
the circuits DTC, PDC, PHC and QDC and driving and controlling the
solid pickup element SP. In addition to this, the operation and
control circuit DCC detects the object distance by the digital
correlation method as described above based on the quantized
picture image data QD from the quantizing circuit QC. The operation
and control circuit DCC also compares the lens position data from
the lens position data output means LPD with the obtained object
distance data to output lens control signals through the output
terminals MU1, MU2, ML1 and ML2. The output signals from the output
terminals MU1, MU2, ML1 and ML2 are, for example, as shown in Table
1 below:
TABLE 1 ______________________________________ Condition Terminal
Near-Focus In-Focus Far-Focus
______________________________________ MU1 L L H MU2 H L L ML1 L H
H ML2 H H L ______________________________________ H: High Level;
L: Low Level
As an example of the specific construction of the operation and
control circuit DCC having these functions, there may be adopted
the circuit construction as shown in FIG. 9 of U.S. patent
application Ser. No. 121,690 filed on Feb. 15, 1980, now U.S. Pat.
No. 4,329,033, of the present assignee. Similarly, the construction
as shown in FIGS. 3, 7A and 7B of this U.S. patent application may
be adopted as the construction for the solid pickup element SP and
the circuits DTC, DA, PDC, PHC, RDC and QC.
The mode of operation of the circuit shown in FIG. 3 will be
described with reference to the timing chart shown in FIG. 4.
Referring to FIG. 4, focus detection data is periodically output
from the control signal output terminals MU1, MU2, ML1 and ML2 of
the operation and control circuit DCC at times t9, t20, . . . and
is held until the next focus detection datum is output.
During the period of time t1 to t9, it is assumed that the
photographic lens LS is set at the in-focus point according to the
focus detection data obtained before time t1. In this case, since
the output terminals MU1 and MU2 are at low level and the output
terminals ML1 and ML2 are at high level, the transistors Tr3 and
Tr4 are turned on, the transistors Tr1 and Tr2 are turned off, and
the two ends of the motor MT are short-circuited to stop the motor
MT. Referring to FIG. 4, symbol .phi.D denotes a pulse for dark
current signal detection by the solid pickup element SP and for
resetting the peak value detecting circuit, .phi.P denotes a
picture element range specifying pulse for detection of the peak
value, .phi.H denotes a peak value holding pulse, .phi.B denotes a
resetting pulse for resetting the flip-flops FP1 and FP2 in the
quantized condition judging circuit QDC, .phi.A denotes a picture
element range specifying pulse for judging the quantized condition
for the AND gates AN1 and AN2, and .phi.C denotes a latch pulse for
the flip-flop FP3. These pulses are all output from the operation
and control circuit DCC.
When the contrast of the object becomes low under the in-focus
condition of the photographic lens LS as shown by DA-OUT in the
period from time t2 to t6, i.e., the output from the differential
amplifying circuit DA, the effects of the non-uniformity of the
solid pickup element SP become predominant and erratic focus
detection data may be output. However, even under such a condition,
if signals are input which are not of single level, that is, if
data including signals of "1" or "0" are input as the quantized
picture element data of the images of the reference field of view
and the actual field of view, the Q outputs of the flip-flops FP1
and FP2 are inverted to high level to thereby hold the signals of
high level at the flip-flop FP3. Then, the Q output of the
flip-flop FP3 becomes high level, and the output from the terminal
SLF becomes low level. Then, the motor drive circuit MDC controls
the motor MT in response to the control signals from the output
terminals MU1, MU2, ML1 and ML2 of the operation and control
circuit DCC. It is now assumed that the operation is performed in
the period of time t3 to t9, the erratic focus detection data due
to an object of relatively low contrast is output at time t9, and
the output at the terminal ML1 is then inverted to low level and
the output terminal MU2 is inverted to high level. Then,
transistors Tr4 and Tr8 are turned off. The delay capacitor C2 for
preventing unstable operation starts charging to turn on the
transistor Tr6 at a time when the stored charge reaches the
threshold value VTH of the transistor Tr6. A current flows to the
motor MT through the transistors Tr2 and Tr3, and the photographic
lens LS begins to be driven in an erroneous direction. When the
contrast of the object becomes very low during the period of time
t12 to t16, the quantized data become signals of single level
alone. Since new focus detection data may not be obtained under
this condition, the erroneous focus detection datum output at time
t9 remains output. The photographic lens LS is thus driven in an
erroneous direction and the image is blurred. In order to solve
this problem, according to the present invention, the quantized
condition judging circuit QDC is incorporated so that driving of
the photographic lens LS is interrupted and the erratic operation
which might otherwise result may thus be prevented when the
quantized data become signals of single level. Hence, the scanning
picture element signals for the object of the low contrast pattern
as in the period of time t12 to t16 are all converted to quantized
data of high level by the quantizing circuit QC. Although the Q
output of the flip-flop FP2 may be inverted to high level at the
quantized condition control circuit QDC, the output of the AND
circuit AN1 remains at low level. Therefore, since the output of
the AND circuit AN3 remains at low level, the flip-flop FP3 holds
the signals of low level at time t13 so that the Q output of the
flip-flop FP3 may become low level and the output from the terminal
SLF of the quantized condition judging circuit QDC may be inverted
to high level. Then, at the lens control invalidating circuit IEC,
the transistor Tr11 is turned on through the diode D7, the
transistors Tr10 and Tr9 are turned off, and the supply of power to
the motor drive circuit MDC is interrupted. Simultaneously, since
the transistors Tr3 and Tr4 are turned on through the diodes D5 and
D6, the ends of the motor MT are short-circuited at the ground side
and the driving of the photographic lens LS is instantaneously
interrupted. In this manner, even when the contrast of the object
is low and erroneous focus detection data is output from the
operation and control circuit DCC, if the contrast becomes very
low, the driving of the photographic lens LS is interrupted by the
operation of the quantized condition judging circuit QDC, thus
preventing the erratic operation of the system which may otherwise
be caused by the low contrast. In this case, it is advantageous to
set the time constant of the delay circuit for preventing the
unstable operation (delay capacitors C1 and C2) of the motor drive
circuit MDC to be slightly longer (e.g., to a time period
corresponding to two cycles under the maximum integrating time of
the solid pickup element SP) than the operating period of the
quantized condition judging circuit QDC, that is, the operating
period of the system.
The resistors R3, R5, R8 and R10 are current limiting resistors
which are not necessarily required if constant current signals are
output through the output terminals MU1, MU2, ML1 and ML2 from the
operation and control circuit DCC. Referring to FIG. 4, the period
of time t9 to t10 is a resetting period of the extraneous charge on
the solid pickup element SP, and the period of time t10 to t11 is a
storing period for the picture image data corresponding to the
object.
In the above embodiment, description has been made with reference
to the case wherein the picture image data is quantized on the
basis of the peak value Vp of the picture image data VF
corresponding to the image in the reference field of view. However,
similar advantageous effects may be obtained if the quantized data
is obtained by quantizing differentiated values of the picture
image data corresponding to the object according to a constant
reference voltage (threshold voltage).
In the above embodiment, the quantization of the image data is
performed according to a single reference value (threshold level)
based on the peak value Vp of the picture image data VF
corresponding to the image of the reference field of view, and the
quantized condition is judged based on the quantized signals.
However, it may be accomplished in the following manner. A
plurality of quantized reference values (threshold levels) are set
according to the peak value Vp of the picture image data
corresponding to the image in the reference field of view, and the
image data are quantized according to the plurality of quantized
reference values. During this procedure, for example, the
computation for the purpose of object distance detection is
performed on the basis of a quantized signal (QD1 in FIG. 3)
according to a first quantized reference value of higher level, and
the quantized condition is judged on the basis of a quantized
signal (QD2 of FIG. 3) according to a second quantized reference
value of lower level. In this manner, the contrast may be more
completely and effectively judged. Moreover, correct quantization
of the picture image data may be expected when the contrast
judgment is good, so that correct focus detection data may be
obtained with excellent reliability.
Furthermore, as shown by the broken lines (a) and (b) in FIG. 3, by
resetting the delay circuit for preventing unstable operation
(capacitors C1 and C2) of the motor drive circuit MDC by signals
from the terminal SLF which are output from the quantized condition
judging circuit QDC, the positioning of the photographic lens LS
may be more stably accomplished.
In the above embodiment, the transistors Tr3 and Tr4 and the lens
control invalidating circuit IEC consisting of the transistors Tr9
to Tr11, the resistors R11 and R12, and the diodes D5 to D7, are
controlled by the signals from the terminal SLF in order to prevent
erratic control of the photographic lens LS. However, the outputs
from the terminals SLF and ML2 and the outputs of the terminals SLF
and ML1 may be received by separate OR gates to control the
transistors Tr3 and Tr4 with the outputs of the respective OR
gates. On the other hand, the inverted signal of the output of the
terminal SLF and the output of the terminal MU2, and the inverted
signal of the output of the terminal SLF and the terminal MU1 may
be received by AND gates to control the transistors Tr5 and Tr6
with the outputs of the respective AND gates. Similar effects may
also be obtained in this case.
Furthermore, in the above embodiment, the quantized condition
judging circuit QDC judges the quantized conditions of the
quantized signals of both images in the reference field of view
(control signal .phi.A in FIG. 4). However, only one of the
quantized signals may be judged. For this purpose, the two periods
of high level of the control signal .phi.A in FIG. 4 may be reduced
to one.
In summary, there is provided according to the improvement offered
by the present invention an automatic continuous focusing device of
the type wherein the image is scanned and the scanning signals are
quantized, for example, an automatic focusing device wherein two
detected images of an object formed by a distance detection optical
system with a relative distance therebetween associated with the
object distance are scanned by a scanning means, the scanning
signals thus obtained are quantized by a quantizing means to
generate quantized picture element data, the relative distance
between the two detected images is detected by operation means on
the basis of the quantized picture element data, and an imaging
optical system to be focused on the object is controlled by a
controlling means. According to this device, unstable operation of
the quantized picture element data and erratic operations are well
prevented, despite a non-suitable quantized condition of the
quantized picture element data due to low contrast of the object or
the like, whereby stable images may be formed constantly with a
camera or the like. This improvement is thus very advantageous for
an automatic focusing device of the type which is subject to
changes in the contrast of the object.
In the lens control invalidating circuit IEC of the embodiment
shown in FIG. 3, the power supply part consisting of the
transistors Tr9 to Tr11, the resistors R11 and R12, and the diode
D7 acts to interrupt supply of power to the drive circuit MDC by an
output of high level from the terminal SLF of the judging circuit
QDC so as not to damage the motor drive circuit MDC when the
transistors Tr3 and Tr4 are rendered conductive by this output of
high level. However, when such damage to the circuit may be
prevented by modification of the overall circuitry, such a power
supply control part may be eliminated. Here the connecting
construction of the diodes D5 and D6 with transistors Tr3 and Tr4
is important for fast control of the motor MT such that the
transistors Tr3 and Tr4 may be rendered conductive by the output of
high level from the terminal SLF of the judging circuit QDC to
ground both ends of the motor MT so as to immediately render the
motor MT inoperative. However, energy saving may be achieved to a
significant extent by incorporating the power supply control part
as described above.
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